Magnesium base alloy



Patented July 10, 1945 UNITED STATES PATENT, OFFICE MAGNESIUM BASE ALLOY Philip '1. Stroup and George F. Sager, New Kensington, Pa., assignors to Aluminum Company of America, Pittsbm-gh, Pa., a corporation of Pennsylvania,

No Drawing. Application July 10, 1942, Serial No. 450,401

Claims.

media containing oxygen. Commercial usefulness? of a metal, such as magnesium, depends not alone upon its essential properties, or those which may be imparted to it by alloying it with lesser quantitles of other metals, but also upon the ease with which the metal, or such alloys, may be remelted, cast, worked or otherwise formed into the various conditions and shapes necess to ultimate use. The propensity oi" magnesium to drstluctivell oxidise when in the molten state is great. Under many conditions, normal to the handling of other molten metals, molten magnesium burns or otherwise reverts to the oxide in very substantial part. When in the solid state, magnesium base. alloys omdize, under some conditions, to a comparatively severe extent. Since extensive handling oi. magnesium and magnesium base alloys in the molten condition is a necessary preliminary to operations designed to shape or work the metal, the w u culties presented by this pronounced tendency to oxidize are encountered in almost every instance and. are universal in the magnesium industry.

Confronted with these problems the industry has devised methods and devices by which to shield molten magnesium and magnesium base alloys from contact with air and moisture, or other deleterious media, during manufacturing operations. One such method is to envelop the molten metal in a protective gas. Another is to constantly protect its exposed surfaces with a salt flux. Other more elaborate methods and devices are frequently necessary. Other means have also been sought to minimize the tendency of magnesium and magnesium base alloys to oxidize and to thus reduce the necessity for the expensive protective measures above mentloned. Calcium has been alloyed with the magnesium for this purpose, and while the magnesium or magnesium base alloy thus alloyed does not oxidize as severely as before the total effect is not sufilclent to do more than supplement the usual protective measures. Better results have been obtained when beryllium has been added to magnesium or magnesium base alloys, it having been found that the eii'ect of beryllium in minimizing the oxidation of magnesium is much greater than that of a corresponding amount or calcium. However, the use of beryllium as an alloying element in the 5 amounts hitherto proposed has created other dimculties. It has been observed that when beryllium is present in a magnesium base alloy, the grain structure of the metal may be undesirably coarsened and the strength of the alloy in substantially reduced. To cure these difllculties ithasbeen suggested that an amount of zirconium be slmultandifsli'added *to themagnesium to prevent coarsening of the grain of that metal by the beryllium. However, zirconium alloys with is dimculty with magnesium and tends tosegregate in the alloys, and in many cases its use is considered expensive. Prior to the present inventionthere was no satisfactory means whereby it was possible to talre advantage of the known no effect oi beryllium in a satisfactory and commercially eiiective way.

We have found that the hitherto unsatisfactory attempts'to use beryllium for the above-described purposes result from previous misconceptions as 2:, to the amount of beryllium to be added to the magnesium or magnesium alloy and that successful use of beryllium depends upon the addition of a carefully controlled and minute amount of beryllium representing between 0.00005 and 0.001 per cent :by weight oi the total alloy. When these amounts are used it is possible to produce magnesium base alloy having a resistance to oxidation f greater than, but grain size and physical proper-.

ties substantially equal to, the same alloy devoid of beryllium. The amounts hitherto suggestedusually about 0.005 to 2.0 per cent by weight-are entirely too great, The minute amounts used in accordance with our invention are, despite the fact that they are oi an order usually disregarded as a "trace" of an element, not present in primary magnesium metal and must be positively added thereto. Where secondary magnesium metal or alloy is available and contains beryllium in larger amount, it may of course be diluted with other magnesium to reduce the amount of beryllium in composed because in this way the amount of beryllium added can be readily controlled. However, a berylliumsalt decomposable at the alloying temperature to produce metallic beryllium may be also used as may any other means which affords control of the amount of beryllium finally present in the mixture.

While the benefits of our invention will be realized in substantial part when the beryllium I is present in the magnesium in an amount which is between-0.00005 and 0.001 per cent by weight of the total alloy, we prefer in practice to add beryllium in amounts between 0.0001 and 0.001 per cent by weight, and by the use of amounts within this rangewe have obtained excellent commercial results. The reasons why the use of beryllium as ananti-oxidant in magnesium and magnesium base alloys has not been previously successful-are more readily understoodwhen it is realized that an addition of as small amount of beryllium as 0.0025 per cent by weight (prior workers have not suggested less than about 0.005 per cent) may decrease the tensile strength of a commercial magnesium base alloy by asgmuch as 4000 pounds per square inch, which is often a decrease of 10 per cent or more in total strength, and the elongation by as much as 25 per cent of the total. Such an amount of beryllium in one instance reduced the number of microscopically observable metal grains per square millimeter of observed surface from about 100 to 2, which reduction is so great as to be a positive qualitative measure of a very substantial grain-coarsening efiect. However, the addition of amounts of beryllium in accordance with the precepts of our invention often results in no decrease in either strength or elongation of the magnesium alloy and has not been observed, to cause substantially harmful efiects of this nature at any time. Moreover, the presence of beryllium in amounts between 0.00005 and 0.001 per cent by weight leads to no substantial coarsening of grain of the magnesium alloy ,as evidenced by the fact that the microscopically observable effect is usually of the order of a reduction of from, say, 100 observable grains per mm." of observed surface to 50 to 90 grains.

The beneficial eflects of the additionv of beryllium to magnesium in amounts in accordance with this invention are obtained in the various magnesium base alloys formed by adding to magnesium amounts of such alloying elements as aluminum, manganese, zinc, cadmium, silicon, copper, silver, cerium, calcium, zirconium and other elements usable or used in commercial magnesium base alloys.

The practice of this invention is particularly useful and beneficial in connection with magnesium base alloys containing about 2 to about 12 per cent by weight of aluminum either as the sole added alloying element (apart from the beryllium) or as but one of the alloying ele-- ments present. These alloys are very suscep- 2,aso,aoo

tible to grain coarsening caused by the presence of beryllium in the amounts used prior to this invention, and the benefits obtained by the addition and presence of the minute amounts of beryllium postulated by this invention are particularly striking. n the other hand the binary magnesium-manganese alloys, such as those con-' taining 1 to 2 per cent of manganese, are generally coarse grained and the addition to them of beryllium, even in larger amounts than are contemplated by our invention, does not appreciably affect their grain structure. The amounts of beryllium which we add in accordance with our invention, however, may be provided to increase the resistance of these alloys to oxidation. Mention of these specific alloys is but illustrative of magnesium base alloys in the practice of this invention. The invention is limited only as expressed in the appended claims and is not intended to be limited to these speciflc alloys. The term magnesium base alloys" as used herein and in the appended claims is intended to include alloyscontaining more than 50' per cent by weight of magnesium which exhibit, as most of them do, the pronounced oxidizing tendencies of pure magnesiumw We claim:

.1. The processeofhandling molten magnesium and magnesium basealloy which consists in increasing the resistance to oxidation of the molten metal by providing, in the metal an amount of beryllium equal to between 0.00005 and 0.001 per cent by weight of the total composition.

2. The process of handling molten magnesium and magnesium base alloy which consists in increasing the resistance to oxidation of the molten metal by providing in the metal an amount of beryllium equal to between 0.0001 and 0.001 per 40 cent by weight of the total composition.

3. Magnesium base alloy containing beryllium in amount between 0.00005 and 0.001 per cent by weight of the total alloy and characterized by a resistance to oxidation greater than, and grain structure and physical properties substantially equal to, that of the same alloy devoid of beryllium.

4. Magnesium base alloy containing about 2 to about 12 per cent by weight of aluminum and beryllium in amount between 0.00005 and 0.001 per cent by weight and characterized by a resistance to oxidation greater than, and grain structure and physical properties substantially equal to, that of the same alloy devoid of beryllium.

5. Magnesium base alloy containing about 2 to about 12 per cent by weight of aluminum and beryllium in amount between 0.0001 and 0.001- per cent by weight and characterized by a resistance to oxidation greater than, and grain structure and physical properties substantially equal to, that of the same alloy devoid of beryllium.

PHIIIJP T. BTROUP.

GEORGE F. SABER. 

